(a) Technical Field
The present invention relates to a system and a method for controlling the thrust force of a belt of a continuously variable transmission (CVT) that can prevent the belt from slipping and improve fuel efficiency.
(b) Related Art
In case of a CVT, speed ratios are continuously changed within a specific range. Currently, a CVT is mainly used in hybrid vehicles. According to typical hybrid vehicles, a first planetary gear set is disposed on a first shaft, a second planetary gear set is disposed on a second shaft disposed in parallel to the first shaft, and a third shaft disposed in parallel to the second shaft is connected to an output shaft.
An engine is mounted on the first shaft and is connected to one operating member of the first planetary gear set, and the first motor/generator is mounted on the first shaft and is connected to another operating member of the first planetary gear set. In addition, the second motor/generator is mounted on the second shaft and is connected to one operating member of the second planetary gear set, and the third shaft is connected to another operating member of the second planetary gear set.
Further, the other operating members of the first and second planetary gear sets are respectively connected to first and second pulleys, and the first and second pulleys are connected to each other by a belt such that torque of the first shaft is transmitted to the second shaft. Accordingly, a shift to a target speed ratio is continuously performed by using engine torque and motor torque.
Since torque is transmitted by frictional force between the belt and the pulley, it is important to predict the force that the belt applies to the pulley (thrust force). That is, if the thrust force is excessive, power delivery efficiency and fuel mileage may be reduced.
In addition, even if a vehicle drives with a suitable thrust force, road torque may be transmitted from the output shaft when the vehicle is running on a rough road, thereby causing belt slip.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.